1. Field of the Invention
This invention relates to a process for the continuous solid phase polymerization of polyester. More particularly, the invention relates to a process for the continuous production of high molecular weight polyethylene terephthalate by polycondensation in the solid phase wherein granulated polyethylene terephthalate is first crystallized and thereafter subjected to solid state polycondensation.
2. The Prior Art
High viscosity polyesters have been produced in the solid phase at temperatures below the melting point of the polyethylene terephthalate condensates and under an inert gas blanket or vacuum. However, this known method has the disadvantage that the granulate becomes sticky in the course of the solid state polycondensation. Rothe et al U.S. Pat. No. 4,064,112 discloses prior attempts to solve this sticking problem, including that for which the patent was issued. The use of rotary vacuum units or fluidized bed units overcame the sticking problem by constant agitation of the polymer during the polymerization process but were economically disadvantageous.
The Rothe et al patent is directed to a method of avoiding sticking without agitation in the reaction stage, but with definite limits on operating temperatures. The polymerization temperature must always be equal to or less than the crystallization temperature. That is, the Rothe invention is based on the "surprising discovery" that sticking of the granulate during the solid state polycondensation may be completely prevented if the solid state polycondensation is preceded by a crystallization utilizing temperatures which are higher than, or equal to, the temperature at which the solid state polycondensation is effected. Agitation of the granulate is required only within the crystallization zone. Crystallization according to Rothe et al is effected at a temperature of 220.degree. C. to 260.degree. C. and the solid state polycondensation is effected at the same or lower temperature within the same range.